Satellite controlled power control for personal communication user terminals

ABSTRACT

A system and method for controlling the transmission power of a user terminal in a satellite communications system of a type that includes a ground segment, comprised of at least one user terminal and at least one terrestrial gateway, and a space segment, comprised of a plurality of satellites in a non-geosynchronous earth orbit. The method includes the steps of (a) transmitting an uplink signal from the user terminal simultaneously to at least two satellites of the space segment and (b) receiving the uplink signal with each of the at least two satellites. The method further includes the steps of (c) determining, in the space segment, a difference value representing a difference between a received signal strength indication and a desired received signal strength indication for each of the at least two satellites; (d) in response to the difference value, generating in the space segment at least one power control command for use by the user terminal; (e) transmitting the at least one generated power control command from the space segment to the user terminal; and (f) adjusting a transmitted power of the uplink signal in accordance with the at least one power control command.

FIELD OF THE INVENTION

This invention relates generally to satellite communications systemsand, in particular, to RF power control techniques in bothgeosynchronous orbit (GSO) and non-geosynchronous orbit (non-GSO)satellite communications systems.

BACKGROUND OF THE INVENTION

Satellite telephone systems for fixed and mobile communications areemerging as a new global business. These systems utilize many individualcircuits routed through one satellite or a constellation of manysatellites to effect communications. The value of the satellitetelephone system is that it provides ubiquitous coverage of large areasof the earth without the construction of many small terrestrial cells.Since the allocation of frequencies for satellite services, a number ofproposals have been advanced for the deployment of satellitecommunications systems. In general, these proposals have involved eithera Time Division Multiple Access (TDMA) technique or a Code DivisionMultiple Access (CDMA) technique.

In addition, there have been two general proposals advanced for thesatellite operation itself. The first proposal is for an onboardprocessing design which involves processing and multiplexing, forfeederlink bandwidth reduction, on the satellite itself. Onboardprocessing involves reducing an uplinked communications signal tobaseband (i.e., to digital bits), and then possibly switching thesignal, via inter-satellite links, to another satellite for downlinking.The second proposal uses a "bent" pipe satellite transponder as aclassical repeater to receive, frequency shift, and transmit (repeat)signals without any processing on the satellite or any reduction of thesignals to baseband.

With the first type of system (i.e., the onboard processing system) aterrestrial gateway, which functions as a ground insertion point to thePublic Switched Telephone Network (PSTN), may be located at anyarbitrary place. Onboard processing of signals has many advantages whichcan be traded off against the simplicity of the "bent pipe" repeater.One significant advantage of onboard processing is that the usercommunications traffic signals (e.g., voice and/or data), and anyrequired signaling for control of user terminals and other devices, isestablished and performed o n the satellite. Furthermore, as in CDMAsystems, self-interference can be avoided on the down links, therebyincreasing the capacity of the system. Further it can be appreciatedthat routing of signals between various satellite node points can beeffected, through inter-satellite links, thereby allowing a significantamount of flexibility in call connection. Finally, conservation ofspectrum can be effected by utilizing that portion of the spectrum whichmay be unused due to the system inefficiency of the "bent pipe"architecture.

It should thus be appreciated that an advantage of on-board processingis that signals arriving at or being sent from the satellite may becontrolled on the satellite by information received at the satellite, oras relayed from the satellite to a control point. Typically this controlpoint has been a ground station.

It should thus be further appreciated that this latter techniquerequires that the control point (e.g., ground station) must be made morecomplex in order to participate in the control loop. It would bedesirable, then, to simplify the overall system and control pointcomplexity.

Reference in this regard can be had to the following U.S. Pat. Nos.:4,991,199, Saam, "Uplink Power Control Mechanism For MaintainingConstant Output Power From Satellite Transponder"; 4,752,967, Bustamanteet al., "Power Control System For Satellite Communications"; 5,339,330,Mallinckrodt, "Integrated Cellular Communications System"; 4,752,925,Thompson et al., "Two-Hop Collocated Satellite Communications System";5,126,748, Ames et al., "Dual Satellite Navigation System And Method";5,109,390, Gilhousen et al., "Diversity Receiver In A CDMA CellularTelephone System"; and 5,138,631, Taylor, "Satellite CommunicationNetwork".

Reference can also be had to commonly assigned and allowed U.S. patentapplication: Ser. No.: 08/467,209, filing date: Jun. 06, 1995, entitled"Closed Loop Power Control For Low Earth Orbit Satellite CommunicationsSystem", by Robert A. Wiedeman and Michael J. Sites.

OBJECT OF THE INVENTION

It is an object of this invention to provide a power control techniquewhich applies control autonomously from a satellite system, thussimplifying the ground station and reducing system complexity.

SUMMARY OF THE INVENTION

The foregoing and other problems are overcome and the object of theinvention is realized by methods and apparatus in accordance withembodiments of this invention.

This invention pertains to satellite communications systems that includenon-GSO satellites. The invention employs power control loops whichsatisfy the need to individually control the power of each user terminalby means of satellite on-board processing, and not with a central,ground-based gateway earth station.

In accordance with an aspect of this invention there is disclosed amethod for controlling the transmission power of a user terminal in asatellite communications system of a type that includes a groundsegment, comprised of at least one user terminal and at least oneterrestrial gateway, and a space segment, comprised of a plurality ofsatellites. The method includes the steps of (a) transmitting an uplinksignal from the user terminal simultaneously to at least two satellitesof the space segment and (b) receiving the uplink signal with each ofthe at least two satellites. The method further includes the steps of(c) determining, in the space segment, a difference value representing adifference between a received signal strength indication and a desiredreceived signal strength indication for each of the at least twosatellites; (d) in response to the difference value, generating in thespace segment at least one power control command for use by the userterminal; (e) transmitting the at least one generated power controlcommand from the space segment to the user terminal; and (f) adjusting atransmitted power of the uplink signal in accordance with the at leastone power control command.

BRIEF DESCRIPTION OF THE DRAWINGS

The above set forth and other features of the invention are made moreapparent in the ensuing Detailed Description of the Invention when readin conjunction with the attached Drawings, wherein:

FIG. 1 is a simplified diagram that illustrates a relationship of anon-geosynchronous orbit (NGSO) satellite constellation and a GSOsatellite constellation with respect to the earth and to one another;

FIG. 2 is a more detailed view of FIG. 1 and shows various userterminals, non-GSO gateways, GSO gateways, and various communicationslinks between these components;

FIG. 3A illustrates a case where two user terminals are operated innon-overlapping NGSO satellite coverage regions;

FIG. 3B illustrates a case where a user terminal is operated in anoverlapped area between two NGSO satellite coverage regions;

FIG. 3C is a simplified block diagram of a user terminal;

FIG. 3D is a simplified block diagram of one of the NGSO satellites;

FIG. 4 shows in further detail the overlapped coverage area case of FIG.3B;

FIG. 5 shows in still further detail the overlapped coverage area caseof FIGS. 3B and 4, and further illustrates in simplified block diagramform one of the GSO satellites;

FIG. 6 is a simplified block diagram of circuitry suitable for reducinga received user transmission to baseband; and

FIG. 7 is a logic flow diagram in accordance with a method of thisinvention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, and by way of introduction, in accordance with theteaching of this invention there is described a technique forsupplementing a first or primary NGSO satellite constellation, such as alow earth orbit (LEO) satellite constellation, with a secondary, higherorbit constellation (Medium Earth Orbit (MEO) and/or GSOconstellation(s)) so as to improve the control over user terminal andsatellite power consumption. The following U.S. Patents teach variousaspects of a LEO satellite constellation, and the associatedcommunication system: U.S. Pat. No.: 5,552,798, issued Sep. 03, 1996,entitled "Antenna for Multipath Satellite Communication Links", by F. J.Dietrich and P. A. Monte; U.S. Pat. No.: 5,422,647, issued Jun. 06,1995, entitled "Mobile Communication Satellite Payload", by E.Hirshfield and C. A. Tsao; U.S. Pat. No.: 5,504,493, issued Apr. 02,1996, entitled "Active Transmit Phased Array Antenna with AmplitudeTaper", by E. Hirshfield; U.S. Pat. Nos. 5,448,623, issued Sep. 05,1995, and 5,526,404, issued Jun. 11, 1996, "Satellite TelecommunicationsSystem Using Network Coordinating Gateways Operative with a TerrestrialCommunication System", by R. A. Wiedeman and P. A. Monte; and U.S. Pat.No. 5,233,626, issued Aug. 03, 1993, entitled "Repeater Diversity SpreadSpectrum Communication System", by S. A. Ames. The disclosures of theseU.S. Patents is incorporated by reference herein in their entireties. Aswill be made apparent below, these teachings are modified so as toprovide, in some embodiments, on-board satellite processing of receiveduser terminal transmissions, and inter-satellite links.

In a presently preferred embodiment of this invention the LEO satelliteconstellation 10 includes satellites 10a at an altitude of ≈1400 km ineight circular planes offset by 45°, inclined at 52° relative to theequator with six satellites 10a in each plane (which may be referred toas a Walker constellation). To optimize the effectiveness of thecoverage, path diversity is employed to mitigate against localobstructions such as trees, buildings, and mountains. Path diversityrequires that a user terminal on the ground have a simultaneous view oftwo or more satellites at elevations above about 10° over the horizon.The LEO constellation described above provides multiple satellitecoverage.

FIG. 1 illustrates a general configuration of a satellite system 1 inaccordance with the teaching of this invention. The satellitecommunications system 1 includes a non-geosynchronous orbit (NGSO)satellite or a plurality of NGSO satellites 2, which may be referred tocollectively as a satellite constellation. This constellation may besimilar to that described in foregoing U.S. Patents, although theteaching of this invention should not be read to be limited to only thisparticular type of LEO system. The NGSO satellites 2 orbit the earth inthe non-geosynchronous orbit 12. It is not necessary that there be morethan one satellite 2, however, the preferred configuration contains manysatellites. Each satellite 2 has an associated earth coverage area 21.FIG. 1 also shows as a preferred embodiment a geosynchronous (GSO)satellite 3 which orbits the earth in a synchronous orbit 11. Asynchronous orbit is one in which the satellite 3 does not have apparentmovement with respect to points on the earth. The GSO satellite 3 has anassociated earth coverage area 22 which, because of the difference inaltitude with respect to the non-GSO satellites 2, is significantlylarger than the coverage area 21.

It should be noted that it is not necessary for the satellite 3 to be ina GSO orbit, but in fact may be non-geosynchronous as well. For example,the satellite 3 could be in a medium earth orbit (MEO). Also, there aretypically more than one of the satellites 3 for providing whole earth ornear whole earth support for the NGSO satellites 2. However, in any casethe satellite 3 should be at a greater altitude than the satellites 2.The NGSO constellation and the higher altitude orbiting constellationmay be collectively referred to as the space segment of the satellitecommunications system.

Reference is now made to FIG. 2 for illustrating the various elementsthat link the system 1 together. User terminals 5 and ground stations orgateways (GWs) 7 and 8, which collectively form all or a portion of aground segment of the satellite communications system, are located on ornear the surface of the earth. Within the coverage area 21 of the NGSOsatellites 2 there is at least one, but typically many, user terminals5,, which may be fixed, handheld, or mobile terminals capable of voiceand/or data transmission and reception. The user terminals 5 communicateover links to the NGSO satellites 2 using an uplink 31 and a downlink32. These signals may be routed on the NGSO satellite 2 to a NGSOgateway (GW-NGSO) 8 within the coverage area 21 of the NGSO satellite 2,or may be routed via the GSO satellite 3 to the gateway (GW-GSO) 7within the coverage area 22 of the GSO satellite 3. The links to theGW-NGSO 8 provide connectivity to the public switched network (PSTN) 6and/or to private networks. The user terminals 5 are connected toGW-NGSO 8 via the user links 31 and 32 and the NGSO satellite 2, with anuplink 35 and a downlink 36 between the NGSO satellite 2 and the GW-NSGO8. Alternatively, the user terminals 5 may be connected to themselvesdirectly through the NGSO satellite 2, and not routed to a gateway. Inaddition, the user terminals 5 may be connected to the GW-GSO 7 withinthe coverage area 22 of the GSO satellite 3. The GW-GSO 7 may also beconnected to the PSTN 6 and/or to private networks.

Operating power on a satellite is a valuable resource which must becontrolled, since there is at any given time a finite amount ofavailable power. In general, with a NGSO satellite system, the poweravailable from the constellation of satellites is directly proportionalto a number of communications circuits that may be supported in thebusiest or peak hour, as the peak hour progresses around the earth fromtime zone to time zone. As such, at any instant, the satellites whichare orbiting over an area can deliver a certain amount of communicationscircuits to the area depending on the state of the power system, thenumber of satellites covering the area, and the amount of spectrumavailable to be utilized. If it is assumed that the amount of spectrumis not a limiting factor, then the number of available satellites andthe available amount of power are the two dominant factors.

Reference is now made to FIGS. 3A and 3B for illustrating two differentembodiments of NGSO systems. In constellation A (FIG. 3A), the coverageareas or regions 21 of the NGSO satellites 2 do not substantiallyoverlap, the user terminals 5 within the coverage regions 21 do notcompete for resources from one NGSO satellite, and the power used isdrawn from one satellite at a time for each user terminal. However, inconstellation B (FIG. 3B), the coverage regions 21 substantially overlapone another, the user terminals 5 within the overlapping coverage zonescompete for resources from two or more NGSO satellites 2, and power maybe drawn from more than one satellite 2 at a time for a given userterminal.

For both of the configurations shown in FIGS. 3A and 3B the power costof the links in both directions is important.

Referring briefly to FIG. 3C, the user terminal 5, in mobile andportable configurations, has a battery 5a which supplies power to adigital section 5b, including a user terminal control processor, and anRF section 5c, comprised of a transmitter, a receiver, and related RFsignal handling components. The function of these various sections is toenable the links 31 and 32 to be established and maintained, via antenna5d, for transmitting and receiving voice and/or data communications.

Referring also to FIG. 3D, the NGSO satellite 2 has battery 2a which ischarged from one or more solar panels 2b through a power control unit2c. When the solar panels 2b are not providing power (during eclipse),the operating power for a digital section 2d and RF section 2e must besupplied from the battery 2a, via the power control unit 2c, toestablish and maintain the links 31, 32, 35, 36, 37 and 38 throughappropriate antennas 2f.

In the case of both FIGS. 3C and 3D it is important to carefully controlthe amount of current drawn from the batteries 5a and 2a, and to alsominimize the weight and size of the batteries. For the NGSO satellite 2,it is also important to minimize the weight and size of the solar panels2b, since solar power generators are typically costly to build and tolaunch. Since the cost of launching a satellite is a strong function ofthe weight that will be lifted, the available power in watts andwatt-hours that can be obtained with reasonably sized batteries andsolar panels determines, to a large degree, the financial viability ofthe satellite communications system.

In order to minimize the weight of the battery 5a of the user terminal 5and to minimize the cost and weight of the satellite power system(2a-2c), it is useful to only transmit the minimum power necessary toclose the RF links 31 and 32 to the user terminals 5. Since the links 31and 32 are subject to various impairments, a variable amount of power isnecessary to overcome the impairments. The specific nature of theimpairments depends on the nature of the operation, the type of systemmodulation being transmitted, and the slant range between the userterminal 5 and the satellite(s) 2.

It should be noted that in the NGSO satellite system the slant range isconstantly varying as the satellites 2 move overhead. Some, but not all,of the various impairments that can be experienced include impairmentsdue to foliage absorption and diffraction, impairments due to buildingblockage or other obstructions in any frequency band, and impairmentsdue to rain attenuation in frequency bands above 5 GHz.

Furthermore, certain types of signal modulation operate most effectivelyif all user terminal transmissions are controlled to a certain level,independent of impairments, slant ranges, and other variations. One typeof signal modulation that behaves in this manner is Spread Spectrumutilizing Code Division Multiple Access, or SS/CDMA. In SS/CDMA thesystem goal is to bring all user terminal 5 transmitted up-link signals31 in a certain frequency channel to approximately the same powerdensity after reception by the satellite receive antenna 2. The powercontrol system to keep the user terminals 5 at the same or at theminimum power level, after reception at the satellite antenna, isindependent of the modulation scheme, impairment, or frequency bandchosen.

As an example, consider rain attenuation in the Ka frequency bands of 28GHz in a SS/CDMA system utilizing the configuration shown in FIG. 3B.The principle applies to other frequencies, links, types of impairments,and system modulation techniques.

Reference in this regard is made to FIG. 4. The system attempts to linkthe two NGSO satellites NGSO SAT-1 and NGSO SAT-2 with the user terminal5. As shown in FIG. 4, the user terminal 5 is transmitting a signal at apower P towards the two satellites simultaneously. The signals receivedat the user terminal 5 from the two satellites are coherently combinedin the user terminal 5 to form a single, composite signal. Reference inthis regard can be had to the above-referenced U.S. Pat. No. 5,233,626,issued Aug. 03, 1993, entitled "Repeater Diversity Spread SpectrumCommunication System", by S. A. Ames. A final destination for the signaltransmitted to the two satellites from the user terminal 5 may be theGSO satellite 3, the NGSO gateway 8, the GSO gateway 7 (via the GSOsatellite 3), or another user terminal 5. In any case there is a certainsignal received quality necessary at the NGSO satellites 2 to achieve adesired result at the final destination.

As is shown in FIG. 4, one of the uplinks 31a, and perhaps also thedownlink 32a, are attenuated by a rain cell 33. The received powerP(NGSO SAT-1) at the NGSO SAT-1 is less than the desired level due tothe rain attenuation (it being realized that the NGSO SAT-1 is alsosimultaneously receiving uplinks from other user terminals 5, which mayor may not be impaired). By knowing that this impairment is occurring,and the level of the impairment, the system can compensate for this linkonly (e.g., only the link 31a) while leaving all other user terminalsunaffected. Thus energy is conserved and satellite cost and weight isminimized by selectively power controlling a terminal or terminals on alink-by-link basis.

Conventional approaches, such as those described in the various U.S.Patents mentioned in the Background section of this patent application,for compensating for the attenuation in one of the user links employpower control loops that are controlled from the ground, and may requirea measurement of signals that are emanating from the ground at a remotereceiver located on the ground. This conventional approach has a numberof drawbacks and disadvantages.

In accordance with an aspect of this invention, the system employs powercontrol loops (open and/or closed) which individually control the powerof each user terminal 5 by means of processing performed in, and powercontrol commands issued from, the space segment of the satellitecommunications system.

FIG. 5 illustrates a presently preferred technique to control thetransmission power of the user terminals 5. The power P transmitted fromthe user terminal 5 is received from two uplinks 31a and 31b by twodifferent NGSO satellites NGSO SAT-1 and NGSO SAT-2, respectively. Asnoted before, the signals on each uplink have different strengths due tovarious impairments (e.g., the presence of the raincell 33).

There are two presently preferred embodiments for implementing thisinvention. One embodiment assumes that the fading is reciprocal in thetwo different frequency bands in which the user terminal 5 is receivingand transmitting, thereby only requiring a single compensation link(open loop). The second embodiment individually controls the separateuplinks 31 an downlinks 32 in each frequency band, thereby not requiringany assumption on the fading depth correlation between the uplink andthe downlink (closed loop). It should be noted that the closed loop andopen loop control can be used together. The closed loop controlembodiment is described first, followed by the open loop power controlembodiment.

It should first be noted that there are different embodiments forimplementing the closed-loop power control in accordance with thisaspect of the invention.

A first closed-loop embodiment uses power control of the user terminal 5uplink signal that is implemented on each NGSO satellite 2 and thenrelayed to the GSO satellite 3 over uplinks 33. In this case, assumethat the path 31a is impaired and is received at NGSO SAT-1 at a lowerpower level than the desired power level. At the NGSO SAT-1 the receivedsignal may either be demodulated entirely or demodulated partially, andthe signal strength information extracted. The signal strengthinformation, when the signal is entirely demodulated, can be a measureor indication of bit error rate or frame error rate. For the case wherethe signal is not entirely demodulated, the signal strength informationcan be based on a measure of the received signal strength.

In general, the signal strength information is considered, for thepurposes of this invention, to be indicative of the power and quality ofthe uplink signal 31 as received at a given one of the receiving NGSOsatellites 2. For a direct sequence (DS) CDMA embodiment of thisinvention, it is also desirable to substantially equalize the power ofthe uplink signals 31 received from each of the user terminals 5 thattransmits to a given one of the NGSO satellites 2.

If the signal is completely demodulated and decoded, it is processed bya digital receiver chain 40 as shown in FIG. 6. The digital receiverchain may be conventional in construction and, assuming a DS-CDMAsignal, includes a mixer 40a for mixing the uplink RF signal with alocal oscillator (LO) signal to generate an IF signal, a despreader 40bthat operates in conjunction with a pseudorandom (PN) code generator 40cto correlate with and extract the user terminal's transmission, ademodulator 40d, and a decoder 40e. The output of the decoder 40e is thebaseband signal. In this case, the strength of the demodulated output,or the soft decisions made by the demodulator 40d, may be used as ameasure of the received signal power. If the transmitted signal is to beonly partially regenerated on the satellite 2, any of the blocks 40d and40e may be omitted, and the signal power level at the output of block40b may be used as a measure of received signal strength. It isunderstood that at least some of the circuitry shown in FIG. 6 isreplicated for each user terminal 5 that transmits a differently spreadsignal in the same band of frequencies.

Referring again to FIG. 5, the indication of signal strength of the userterminal 5 obtained as described in FIG. 6 is then transmitted to theGSO satellite 3 by each NGSO satellite 2 over links 33a and 33b. In theGSO satellite 3 the signal strength indications for the uplinks 31a and31b are received with RF unit 3a from antenna 3b and are passed to aprocessor 3c. In the processor 3c the two signal strength indications(or more, depending on the number of NGSO satellites 2 that arereceiving the user terminal's transmission) are compared to one another.That is, the signal strengths from different paths via several NGSOsatellites 2 are compared on the GSO satellite 3. In the example of FIG.5, the uplink signal 31a received via NGSO SAT-1 is found to be impairedrelative to the uplink signal 31b, which does not experience theattenuation due to the raincell 33.

In a preferred embodiment the sum of the signal strength indicationsreceived from NGSO SATs 1 and 2 are compared with some desired thresholdpower or signal strength level. The difference or shortfall S (if any)with respect to this threshold is then transmitted to the NGSOsatellites 2 from the GSO satellite 3 over links 34a and/or 34b. Thisinformation is then sent to the user terminal 5 through downlinks 32aand/or 32b. In response, the digital portion 5b of the user terminal 5may then either increase the transmitted power directed towards NGSOSAT-1 and/or towards NGSO SAT-2 by an amount determined by the shortfallS. If S is found to larger than some preset value, for example 10 dB,then it may be more power efficient for the non-impaired path via NGSOSAT-2 to be increased by, for example, 3 dB, rather than attempting topenetrate the impaired path to NGSO SAT-1 with substantially moretransmitter power.

A second embodiment for closed-loop power control of the user terminal 5uplink signal 31 is implemented on the GSO satellite 3. In thisembodiment the NGSO satellites 2 simply relay in a linear manner theuser terminal's uplink signals 31a directly to the GSO satellite 3,where the GSO satellite 3: (a) receives both relayed uplink signals and,after demodulating same entirely or in part, (b) compares the relayedsignals, (c) calculates the power deficit to achieve a desired linkquality, and (d) issues power control instructions to the user terminal5 via one or more of the NGSO satellites 2.

For this embodiment it is assumed that the GSO satellite 3 is equippedwith some or all of the circuitry shown in FIG. 6, depending on whethercomplete or partial demodulation is to be accomplished.

In a third embodiment of closed-loop power control the power control ofthe user uplink signal is implemented on the NGSO satellite 2 only. Asin the above example, the path 31a is assumed to be impaired and isreceived at the NGSO SAT-1 at a lower level than that desired. Thissignal could either be demodulated entirely or partially and the signalstrength information extracted as described previously for the firstembodiment.

The user terminal 5 is also transmitting its power toward the NGSO SAT-2on link 31b. This link may be impaired as well but, in general, isimpaired less due to the concentration of the raincell 33 in thedirection of link 31a. The link 31b signal is received at the NGSO SAT-2at a higher level than the same signal received at NGSO SAT-1. Thereceived signal may either be demodulated entirely or partially and thesignal strength information extracted as described above.

In this embodiment each NGSO satellite 2 independently determineswhether the signal strength of the respective uplink 31a or 31b shouldto be increased or decreased. Each NGSO satellite 2 sends a signal tothe user terminal 5 to adjust the transmitter power appropriately. In apreferred embodiment, the user terminal 5 adjusts its transmitted signalpower so as to agree with the lesser of the two commanded power levels.Alternatively, the signal strength may be adjusted to some intermediatevalue that lies between the two commanded power levels.

In this embodiment the NGSO satellites 2 may also relay the powercontrol commands as well to the GSO satellite 3 over links 33a and 33b.In this case the GSO satellite 3 functions as a slower longer-delaypower controller. That is, the NGSO satellites 2 relay the power controlcommands to the user terminal 5 in a shorter-delay power control loop(for example, one power control command per frame period (e.g., 20milliseconds)) and then, in a longer power control loop (e.g., once pern frame periods, where n is greater than one), the GSO satellite 3transmits an overriding power control command to the user terminal 5through one or both the NGSO satellites 2. After setting its transmittedpower at the commanded value received from the GSO satellite 3, theshorter-delay power control loop is used to provide adjustments to thelonger term value for the next n frame periods.

The NGSO satellites 2 may also transmit control messages to one anotherthrough inter-satellite links and thus cooperate to determine theappropriate power control command that should be transmitted to the userterminal 5. In this case the GSO satellite 3 is not required. If NGSOinter-satellite links are not used, then the GSO satellite 3 can beemployed to exchange the messages between the NGSO satellites 2 that arereceiving the user terminal's transmissions.

The foregoing description has been presented in the context of powercontrolling the user terminal uplink 31. However, power control of thedownlink 32 cat also be accomplished. In this case the user terminal 5detects that the downlink 32a from NGSO SAT-1 is faded, and that thedownlink 32b from NGSO SAT-2 is being received with a better quality(e.g., larger received signal strength, fewer bit or frame errors,etc.). The user terminal 5 determines the necessary increase in powerfrom NGSO SAT-1 to compensate for the attenuation in the downlink 32a(for example, to equalize it to the downlink 32b), and communicates thisamount either directly to NGSO SAT-l, or to NGSO SAT-2 which relays thepower increase request via the GSO satellite 3 to the NGSO SAT-1 (or ona control signal crosslink to NGSO SAT-1). If the fading is severe, itmay be preferred to instead increase the power on the "good" link by,for example, 3 dB, rather than attempting to use the "bad" link. Thisdecision can be made by comparing the signal levels received from thetwo links 32a and 32b.

Having thus explained the closed-loop embodiments in accordance with anaspect of this invention, the open-loop embodiment will now bedescribed. This embodiment is premised on the assumption that if a closecorrelation can be made between fades in the downlink and the uplink, anopen loop power control scheme can be implemented using the correlation.For example, if it is found that the uplink fades with a ratio R to thefade on the downlink (due to the difference in frequency between theuplink and downlink), this ratio can be used to implement the open-looppower control. By example, the user terminal 5 detects that the downlink32a from NGSO SAT-1 is faded. In response, the user terminal 5autonomously increases the transmitted power in the uplink 31a to NGSOSAT-1 according to the fade observed in the downlink and the ratio R.

It should be noted that the user terminal 5 may distinguish thesetransmissions by the use of different spreading codes and/or sourceidentification bits that are inserted into the data stream. The sametechnique can be used to distinguish the sources of different powercontrol commands for the closed loop embodiment described previously.

Again, if the fading is severe, it may be preferable to increase theuplink power on the non-faded path by, for example, 3 dB, rather thanattempting to use the path that is experiencing the fade. This decisioncan be made by comparing the signal levels received from the two paths.

It should thus be clear that this invention teaches a method, andreferring to FIG. 7, that includes the steps of (A) transmitting anuplink signal from the user terminal simultaneously to at least twosatellites of the space segment; (B) receiving the uplink signal witheach of the at least two satellites; (C) determining, in the spacesegment, a difference value representing a difference between a receivedsignal strength indication and a desired received signal strengthindication for each of the at least two satellites; (D) in response tothe difference value, generating in the space segment at least one powercontrol command for use by the user terminal; (E) transmitting the atleast one generated power control command from the space segment to theuser terminal; and (F) adjusting a transmitted power of the uplinksignal in accordance with the at least one power control command.

Although described above primarily in the context of a space segmenthaving low earth orbit and geosynchronous earth orbit satellites, itshould be realized that the teaching of this invention applies as wellto a satellite communications system that includes low earth orbit andmedium earth orbit satellites, as well as to medium earth orbit andgeosynchronous earth orbit satellites.

Thus, while the invention has been particularly shown and described withrespect to preferred embodiments thereof, it will be understood by thoseskilled in the art that changes in form and details may be made thereinwithout departing from the scope and spirit of the invention.

What is claimed is:
 1. A method for controlling the transmission powerof a user terminal in a satellite communications system of a type thatincludes a ground segment comprised of at least one user terminal, and aspace segment comprised of a plurality of satellites in anon-geosynchronous earth orbit, comprising the steps of:transmitting anuplink signal from the user terminal simultaneously to at least twosatellites of the space segment; receiving the uplink signal with eachof the at least two satellites; determining in the space segment adifference value representing a difference between a received signalstrength indication and a desired received signal strength indicationfor each of the at least two satellites; in response to the differencevalue, generating in the space segment at least one power controlcommand for use by the user terminal; transmitting the at least onegenerated power control command from the space segment to the userterminal; and adjusting a transmitted power of the uplink signal inaccordance with the at least one power control command; wherein the stepof receiving the uplink signal includes a step of relaying the receiveduplink signal to a third satellite, and further comprising the steps ofdemodulating or partially demodulating the relayed uplink signal in thethird satellite to determine the difference value, and wherein the stepof generating the at least one power control command is performed by thethird satellite.
 2. A method as set forth in claim 1, wherein the stepof receiving the uplink signal includes a step of demodulating thereceived uplink signal to baseband.
 3. A method as set forth in claim 1,wherein the step of receiving the uplink signal includes a step ofdespreading the received uplink signal with a spreading code assigned tothe user terminal.
 4. A method as set forth in claim 1, wherein the stepof receiving includes a step of transmitting the received uplink signalto a terrestrial gateway for connection to a terrestrial communicationsnetwork.
 5. A method for controlling the transmission power of a userterminal in a satellite communications system of a type that includes aground segment comprised of at least one user terminal, and a spacesegment comprised of a plurality of satellites in a non-geosynchronousearth orbit, comprising the steps of:transmitting an uplink signal fromthe user terminal simultaneously to at least two satellites of the spacesegment; receiving the uplink signal with each of the at least twosatellites; determining in the space segment a difference valuerepresenting a difference between a received signal strength indicationand a desired received signal strength indication for each of the atleast two satellites; in response to the difference value, generating inthe space segment at least one power control command for use by the userterminal; transmitting the at least one generated power control commandfrom the space segment to the user terminal; and adjusting a transmittedpower of the uplink signal in accordance with the at least one powercontrol command, wherein the steps of generating and transmitting the atleast one power control command occur at first intervals, and furthercomprising a step of generating an override power control command andtransmitting the generated override power control command to the userterminal at second intervals, wherein the second intervals are longerthan the first intervals.
 6. A method as set forth in claim 5, whereinthe step of determining is accomplished in each of the at least twosatellites, and further including a step of transmitting the determineddifference value from each of the at least two satellites to a thirdsatellite in a higher orbit.
 7. A method as set forth in claim 6,wherein the third satellite is in one of a medium earth orbit or ageosynchronous earth orbit.
 8. A method as set forth in claim 6, whereinthe step of generating the at least one power control command isperformed by the third satellite.
 9. A method as set forth in claim 5,wherein the power control commands that are generated and transmitted atthe first intervals are generated by the at least two satellites of aconstellation of low earth orbit or medium earth orbit satellites, andwherein the override power control command is generated and transmittedby a third satellite in a higher altitude earth orbit than the at leasttwo satellites.
 10. A method as set forth in claim 5, wherein the stepof receiving the uplink signal includes a step of demodulating thereceived uplink signal to baseband.
 11. A method as set forth in claim5, wherein the step of receiving the uplink signal includes a step ofdespreading the received uplink signal with a spreading code assigned tothe user terminal.
 12. A method as set forth in claim 5, wherein thestep of receiving includes a step of transmitting the received uplinksignal to a terrestrial gateway for connection to a terrestrialcommunications network.
 13. A satellite communications system of a typethat includes a ground segment, comprised of at least one user terminaland at least one terrestrial gateway, and a space segment, comprised ofa plurality of satellites in a non-geosynchronous earth orbit, andfurther comprising:a transmitter in said user terminal for transmittingan uplink signal simultaneously to at least two satellites of said spacesegment; a receiver in each of the at least two satellites for receivingthe uplink signal; a first controller in said space segment fordetermining a difference value representing a difference between areceived signal strength indication and a desired received signalstrength indication for each of the at least two satellites; a secondcontroller in said space segment that is response to the differencevalue for generating at least one power control command for use by saiduser terminal; a transmitter in said space segment for transmitting theat least one generated power control command to the user terminal; andmeans in said user terminal for adjusting a transmitted power of theuplink signal in accordance with the at least one power control command;wherein said receiver is coupled to a transmitter for relaying thereceived uplink signal to a third satellite, said third satellitecomprising means for demodulating or partially demodulating the relayeduplink signal, and further comprising said first and second controllers.14. A system as set forth in claim 13, wherein said receiver includescircuitry for demodulating the received uplink signal to baseband.
 15. Asystem as set forth in claim 13, wherein said receiver includescircuitry for despreading the received uplink signal with a spreadingcode assigned to the user terminal.
 16. A system as set forth in claim13, wherein the third satellite is in one of a medium earth orbit or ageosynchronous earth orbit.
 17. A system as set forth in claim 13,wherein said receiver has an output coupled to a transmitter fortransmitting the received uplink signal to a terrestrial gateway forconnection to a terrestrial communications network.
 18. A satellitecommunications system of a type that includes a ground segment,comprised of at least one user terminal and at least one terrestrialgateway, and a space segment, comprised of a plurality of satellites ina non-geosynchronous earth orbit, and further comprising:a transmitterin said user terminal for transmitting an uplink signal simultaneouslyto at least two satellites of said space segment; a receiver in each ofthe at least two satellites for receiving the uplink signal; a firstcontroller in said space segment for determining a difference valuerepresenting a difference between a received signal strength indicationand a desired received signal strength indication for each of the atleast two satellites; a second controller in said space segment that isresponse to the difference value for generating at least one powercontrol command for use by said user terminal; a transmitter in saidspace segment for transmitting the at least one generated power controlcommand to the user terminal; and means in said user terminal foradjusting a transmitted power of the unlink signal in accordance withthe at least one power control command, wherein said first and secondcontroller are located in each of said at least two satellites andgenerate and cause to be transmitted, at first intervals, the at leastone power control command, and further comprising, in a satellite thatorbits at a higher altitude than said at least two satellites, a furtherfirst and second controller for generating an override power controlcommand that is transmitted to said user terminal at second intervals,wherein the second intervals are longer than the first intervals.
 19. Asystem as set forth in claim 18, and further including means fortransmitting the determined difference value between each of the atleast two satellites directly or through at least one other satellite.20. A system as set forth in claim 18, wherein the at least twosatellites are a portion of a constellation of low earth orbitsatellites, and wherein the third satellite in one of a medium earthorbit or a geosynchronous earth orbit.
 21. A system as set forth inclaim 18, wherein said receiver includes circuitry for demodulating thereceived uplink signal to baseband.
 22. A system as set forth in claim18, wherein said receiver includes circuitry for despreading thereceived uplink signal with a spreading code assigned to the userterminal.
 23. A satellite communications system, comprising:a pluralityof user terminals each having a transmitter and a receiver; at least onegateway bidirectionally coupled to a terrestrial communications network;a plurality of satellites in earth orbit, at least some of saidsatellites comprising means for receiving communication signals fromsaid user terminals over communication paths and means for transmittingsaid received communications signals to said at least one gateway; and apower control function distributed in said plurality of satellites forgenerating transmitter power control commands individually for saidplurality of user terminals for compensating individual ones of saiduser terminals for communication path impairments; wherein saidtransmitter power control commands are generated at first intervals andtransmitted to respective ones of said user terminals, and furthercomprising at least one satellite that orbits at a higher altitude thansaid satellites that comprise means for receiving communication signalsfrom said user terminals and for transmitting said receivedcommunications signals to said at least one gateway, said at least onesatellite generating an override power control command that istransmitted to respective ones of user terminals at second intervalsthat are longer than the first intervals.
 24. A satellite communicationssystem of a type that includes a ground segment, comprised of at leastone user terminal and at least one terrestrial gateway, and a spacesegment, comprised of a plurality of satellites in a non-geosynchronousearth orbit, and further comprising:a transmitter in said user terminalfor transmitting an uplink signal simultaneously to at least twosatellites of said space segment; a receiver in each of the at least twosatellites for receiving the uplink signal; a first controller in saidspace segment for determining a difference value representing adifference between a received signal strength indication and a desiredreceived signal strength indication for each of the at least twosatellites; a second controller in said space segment that is responseto the difference value for generating at least one power controlcommand for use by said user terminal; a transmitter in said spacesegment for transmitting the at least one generated power controlcommand to the user terminal; and means in said user terminal foradjusting a transmitted power of the uplink signal in accordance withthe at least one power control command; wherein said receiver is coupledto a transmitter for relaying the received uplink signal to a thirdsatellite, said third satellite comprising means for demodulating orpartially demodulating the relayed uplink signal, wherein said firstcontroller that determines the difference value is located in each ofthe at least two satellites, and wherein said second controller islocated in said third satellite.
 25. A system as in claim 24, andfurther including means for transmitting the determined difference valuefrom each of the at least two satellites and said third satellite.
 26. Asystem as in claim 24, wherein the third satellite in one of a mediumearth orbit or a geosynchronous earth orbit.